Subject: Re: [PATCH bpf-next 10/12] tun: run XDP program in tx path

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 23 Jan 2020 03:23:22 -0500

On Wed, Jan 22, 2020 at 06:42:08PM -0700, David Ahern wrote:
> From: Prashant Bhole <[email-masked]>
> 
> Run the XDP program as soon as packet is removed from the ptr
> ring. Since this is XDP in tx path, the traditional handling of
> XDP actions XDP_TX/REDIRECT isn't valid. For this reason we call
> do_xdp_generic_core instead of do_xdp_generic. do_xdp_generic_core
> just runs the program and leaves the action handling to us.
> 
> Signed-off-by: Prashant Bhole <[email-masked]>
> ---
>  drivers/net/tun.c | 153 +++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 150 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index b6bac773f2a0..71bcd4ec2571 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -130,6 +130,7 @@ struct tap_filter {
>  /* MAX_TAP_QUEUES 256 is chosen to allow rx/tx queues to be equal
>   * to max number of VCPUs in guest. */
>  #define MAX_TAP_QUEUES 256
> +#define MAX_TAP_BATCH 64
>  #define MAX_TAP_FLOWS  4096
>  
>  #define TUN_FLOW_EXPIRE (3 * HZ)
> @@ -175,6 +176,7 @@ struct tun_file {
>  	struct tun_struct *detached;
>  	struct ptr_ring tx_ring;
>  	struct xdp_rxq_info xdp_rxq;
> +	void *pkt_ptrs[MAX_TAP_BATCH];
>  };
>  
>  struct tun_page {
> @@ -2140,6 +2142,107 @@ static ssize_t tun_put_user(struct tun_struct *tun,
>  	return total;
>  }
>  
> +static struct sk_buff *tun_prepare_xdp_skb(struct sk_buff *skb)
> +{
> +	struct sk_buff *nskb;
> +
> +	if (skb_shared(skb) || skb_cloned(skb)) {
> +		nskb = skb_copy(skb, GFP_ATOMIC);
> +		consume_skb(skb);
> +		return nskb;
> +	}
> +
> +	return skb;
> +}
> +
> +static u32 tun_do_xdp_tx_generic(struct tun_struct *tun,
> +				 struct sk_buff *skb)
> +{
> +	struct bpf_prog *xdp_prog;
> +	struct xdp_buff xdp;
> +	u32 act = XDP_PASS;
> +
> +	xdp_prog = rcu_dereference(tun->xdp_egress_prog);
> +	if (xdp_prog) {
> +		skb = tun_prepare_xdp_skb(skb);
> +		if (!skb) {
> +			act = XDP_DROP;
> +			kfree_skb(skb);
> +			goto drop;
> +		}
> +
> +		act = do_xdp_generic_core(skb, &xdp, xdp_prog);
> +		switch (act) {
> +		case XDP_TX:
> +			/* Rx path generic XDP will be called in this path
> +			 */
> +			local_bh_disable();
> +			netif_receive_skb(skb);
> +			local_bh_enable();
> +			break;
> +		case XDP_PASS:
> +			break;
> +		case XDP_REDIRECT:
> +			/* Since we are not handling this case yet, let's free
> +			 * skb here. In case of XDP_DROP/XDP_ABORTED, the skb
> +			 * was already freed in do_xdp_generic_core()
> +			 */
> +			kfree_skb(skb);
> +			/* fall through */
> +		default:
> +			bpf_warn_invalid_xdp_action(act);
> +			/* fall through */
> +		case XDP_ABORTED:
> +			trace_xdp_exception(tun->dev, xdp_prog, act);
> +			/* fall through */
> +		case XDP_DROP:
> +			goto drop;
> +		}
> +	}
> +
> +	return act;
> +drop:
> +	this_cpu_inc(tun->pcpu_stats->tx_dropped);
> +	return act;
> +}
> +
> +static u32 tun_do_xdp_tx(struct tun_struct *tun, struct tun_file *tfile,
> +			 struct xdp_frame *frame)
> +{
> +	struct bpf_prog *xdp_prog;
> +	struct xdp_buff xdp;
> +	u32 act = XDP_PASS;
> +
> +	xdp_prog = rcu_dereference(tun->xdp_egress_prog);
> +	if (xdp_prog) {
> +		xdp.data_hard_start = frame->data - frame->headroom;
> +		xdp.data = frame->data;
> +		xdp.data_end = xdp.data + frame->len;
> +		xdp.data_meta = xdp.data - frame->metasize;
> +
> +		act = bpf_prog_run_xdp(xdp_prog, &xdp);
> +		switch (act) {
> +		case XDP_PASS:
> +			break;
> +		case XDP_TX:
> +			/* fall through */
> +		case XDP_REDIRECT:
> +			/* fall through */
> +		default:
> +			bpf_warn_invalid_xdp_action(act);
> +			/* fall through */
> +		case XDP_ABORTED:
> +			trace_xdp_exception(tun->dev, xdp_prog, act);
> +			/* fall through */
> +		case XDP_DROP:
> +			xdp_return_frame_rx_napi(frame);
> +			break;
> +		}
> +	}
> +
> +	return act;
> +}
> +
>  static void *tun_ring_recv(struct tun_file *tfile, int noblock, int *err)
>  {
>  	DECLARE_WAITQUEUE(wait, current);
> @@ -2557,6 +2660,52 @@ static int tun_sendmsg(struct socket *sock, struct msghdr *m, size_t total_len)
>  	return ret;
>  }
>  
> +static int tun_consume_packets(struct tun_file *tfile, void **ptr_array, int n)
> +{
> +	void **pkts = tfile->pkt_ptrs;
> +	struct xdp_frame *frame;
> +	struct tun_struct *tun;
> +	int i, num_ptrs;
> +	int pkt_cnt = 0;
> +	void *ptr;
> +	u32 act;
> +	int batchsz;
> +
> +	if (unlikely(!tfile))
> +		return 0;
> +
> +	rcu_read_lock();
> +	tun = rcu_dereference(tfile->tun);
> +	if (unlikely(!tun)) {
> +		rcu_read_unlock();
> +		return 0;
> +	}
> +
> +	while (n) {
> +		batchsz = (n > MAX_TAP_BATCH) ? MAX_TAP_BATCH : n;
> +		n -= batchsz;
> +		num_ptrs = ptr_ring_consume_batched(&tfile->tx_ring, pkts,
> +						    batchsz);
> +		if (!num_ptrs)
> +			break;

Can't we avoid looping over the packets in the current case
where there are no xdp programs at all?

> +		for (i = 0; i < num_ptrs; i++) {
> +			ptr = pkts[i];
> +			if (tun_is_xdp_frame(ptr)) {
> +				frame = tun_ptr_to_xdp(ptr);
> +				act = tun_do_xdp_tx(tun, tfile, frame);
> +			} else {
> +				act = tun_do_xdp_tx_generic(tun, ptr);
> +			}
> +
> +			if (act == XDP_PASS)
> +				ptr_array[pkt_cnt++] = ptr;
> +		}
> +	}
> +
> +	rcu_read_unlock();
> +	return pkt_cnt;
> +}
> +
>  static int tun_recvmsg(struct socket *sock, struct msghdr *m, size_t total_len,
>  		       int flags)
>  {
> @@ -2577,9 +2726,7 @@ static int tun_recvmsg(struct socket *sock, struct msghdr *m, size_t total_len,
>  			ptr = ctl->ptr;
>  			break;
>  		case TUN_MSG_CONSUME_PKTS:
> -			ret = ptr_ring_consume_batched(&tfile->tx_ring,
> -						       ctl->ptr,
> -						       ctl->num);
> +			ret = tun_consume_packets(tfile, ctl->ptr, ctl->num);
>  			goto out;
>  		case TUN_MSG_UNCONSUME_PKTS:
>  			ptr_ring_unconsume(&tfile->tx_ring, ctl->ptr,
> -- 
> 2.21.1 (Apple Git-122.3)